The Nervous System contains specialized cells called neurons. The neurons coordinate the actions of a person or animal and the Nervous System. The central Nervous system contains the brain and spinal cord, the peripheral nervous system is made up of sensory neurons and clusters of neurons called ganglia. At a more integrative level, the primary function of the nervous system is to control the body. Because of this consistency, glutamatergic cells are frequently referred to as "excitatory ", and GABAergic cells as "inhibitory neurons". Strictly speaking this is an abuse of terminology it is the receptors that are excitatory and inhibitory, not the neurons but it is commonly seen even in scholarly publications. Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second. Alzheimer disease (AD) is the most common cause of dementia in western civilization. It affects more women than men, and the clinical course generally lasts approximately five years. The younger the individual is at the onset of the disease, the more severe the deficits for the patient. One famous contemporary who suffers from the disease is former U.S. President Ronald Reagan. The cerebral cortex and some other forebrain regions atrophy so severely that the brain may weigh less than 1000g at death. Shrinkage is most pronounced in the frontal and temporal lobes. The insula and the medial part of the temporal lobe tend to demonstrate the highest number of . The the number of plaques the higher the degree of dementia. The disease often causes vacuolization of the subpial layers of the temporal and parietal lobes. The spongy state is associated with neuronal loss and is similar to the effects of Creutzfeldt-Jakob disease. Researchers continue to search for causes and cures for AD. The gene that codes for the B-amyloid protein located on chromosome 21 is implicated in the 20% of patients for whom there is a family history of AD. Head injury has been implicated in 3 to 5 % of AD cases. There is a 70 to 90% decrease in the production of the enzyme that makes acetylcholine. Other neurotransmitter abnormalities have also been implicated.

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The digestive system actually has its own nervous system called the 'enteric nervous system' (ENS) that acts almost entirely independently of the brain. The ENS has roughly as many neurons as the spinal cord (~90 million) contained in plexuses that surround the the entire gastrointestinal tract. These include both sensory as well as motor neurons. Together they detect changes in the gut (as a result of ingesting food/drink) and regulate secretion and motility. Although the ENS is under autonomic influence - e.g. increased parasympathetic tone during sleep - the actual influence is limited. In fact the ENS will continue to function normally without any sympathetic/parasympathetic input at all.